In the field of molecular imaging, a method of detecting target molecules with high sensitivity using optical information has become a mainstay. In the method, an optical coherent tomography (OCT) apparatus capable of acquiring information in a depth direction with high resolution using low interference light is attracting attention.
In order to collect three-dimensional information in the OCT apparatus, it is necessary to scan with a light beam output from a light source called a super luminescent diode (SLD) in horizontal and vertical directions. In the OCT apparatus, a light beam output from an SLD light source via a collimator is split into reference light and measurement light by a beam splitter, and mechanical scanning with the light beam is performed in the horizontal and vertical directions using a two-axis galvanometer mirror with respect to the split measurement light. The scanning measurement light is reflected by each layer of a measurement object on which the light is input via an objective lens and returns to the beam splitter as a drive signal S. In the beam splitter, the measurement light returning as the drive signal merges with the reference light reflected and returned by a movable mirror and is input to a photodiode (PD).
In a signal processing unit of the OCT apparatus, the intensity and time lag of the measurement light are detected on the basis of an interference phenomenon caused by the merging of the measurement light and the reference light to derive a spatial positional relationship (three-dimensional information). The OCT apparatus for acquiring tomographic images using low coherence interference uses time domain optical coherence tomography (TD-OCT) and Fourier domain optical coherence tomography (FD-OCT). FD-OCT is classified into spectral domain optical coherence tomography (SD-OCT) and swept-source optical coherence tomography (SS-OCT). A method using a swept light source in SS-OCT is particularly excellent in a high-speed response and the development of various types of high-speed broadband light sources is accelerating.
Among light deflectors that change a traveling direction of light, a light deflector using a KTN (KTa1-xNbxO3 (0<x<1)) crystal or a KLTN (K1-yLiyTa1-xNbxO3) (0<x<1 and 0<Y<1)) crystal to which lithium is added is different from a galvanometer mirror, a polygon mirror, a MEMS mirror, or the like and is a solid-state element that does not have a movable part (see, for example, PCT International Publication No. WO 2006/137408). The KTN crystal is known as a substance having a large electro-optic effect which greatly changes its refractive index when a relatively low voltage is applied. Furthermore, when Ti or Cr electrodes are used, electric charge can be injected into the KTN crystal. By using an internal electric field generated by the injected charge, it is possible to implement a high-speed wide-angle light deflector. Accordingly, in applications requiring general optical components such as lenses, prisms, and mirrors to operate at a high speed, a KTN light deflector in which the optical components are replaced with KTN crystals can be applied (see, for example, PCT International Publication No. WO 2006/137408).
In recent years, attention has been focused on a medical optical tomographic imaging system using a high-speed swept light source in which a light deflector is incorporated in an external resonator by utilizing the high speed of refractive index control in the KTN crystal or the KLTN crystal. Among the above-mentioned OCT apparatuses, the KTN light deflector is a key device for implementing a high speed and is required to operate stably with a high speed. In particular, it is important to stably obtain a necessary and sufficient maximum deflection angle.
A configuration of a light deflector using a conventional KTN crystal is illustrated in FIG. 9. Electrodes 102 and 103 are formed on an upper surface and a lower surface of a KTN crystal 101. A control voltage is applied from the control voltage source 104 between the two electrodes. Incident light 105 is incident on a left side surface of the KTN crystal 101 and is deflected in the KTN crystal 101 while traveling in a z-axis (optical axis) direction. The light changes its traveling direction in an x-axis direction and is emitted from a right side surface of the KTN crystal 101 as emitted light 106. At this time, a deflection angle θ according to the applied voltage is obtained.
A control signal according to the application of the light deflector is applied from the control voltage source 104. For example, a control signal having a shape of a sinusoidal or sawtooth wave is applied in accordance with the application of the light deflector. In order to obtain an appropriate maximum deflection angle, a drive voltage of about several hundred volts is applied to the KTN crystal 101. However, if the light deflector is controlled only by the drive voltage for causing the deflection, a problem occurs with the increase in the drive speed. That is, there is a problem that an ideal space charge control state is not implemented and the deflection angle is decreased because a movement distance of electrons injected from the electrode by the control signal is shorter than the distance between the electrodes.
To solve this problem, a control method of applying a DC bias voltage to an AC drive voltage, injecting electrons into the KTN crystal, and trapping the electrons in a trap has been proposed. In other words, by applying a DC bias voltage and constantly filling the trap in the crystal with electrons, it is possible to stably generate a distribution or inclination of an electric field in the KTN crystal, and implement stable light deflection for a long period of time (see, for example, Japanese Unexamined Patent Publication No. 2015-142111).
Examples of a drive voltage waveform of a conventional KTN light deflector are illustrated in FIG. 10A and FIG. 10B. FIG. 10A is a voltage waveform in which sinusoidal waves are superimposed on a negative DC bias voltage of a fixed voltage. Also, triangular waves and sawtooth waves can be used instead of the sinusoidal waves. Furthermore, before the application of a drive voltage having an AC voltage superimposed on a DC bias voltage, a DC voltage is applied and electrons are filled in the trap in the crystal in advance. FIG. 10B is a voltage waveform when a DC voltage is applied as a trap filling voltage before a drive voltage is applied (see, for example, Japanese Unexamined Patent Publication. No. 2015-068933).